With the spread of electronic devices such as notebook computers, mobile phones, and digital cameras, the demand for secondary batteries for driving these electronic devices is on the increase. In recent years, these electronic devices have increasingly high power consumption with enhancement of functionalities thereof and have been expected to be reduced in size, and hence improvement in performance of the secondary batteries has been required. Among the secondary batteries, a non-aqueous electrolyte secondary battery (particularly, a lithium-ion secondary battery) can be increased in capacity, and this battery has thus been applied to a variety of electronic devices.
Generally, a non-aqueous electrolyte secondary battery has a configuration in which a positive electrode and a negative electrode are connected via a non-aqueous electrolyte (non-aqueous electrolytic solution), and stored in a battery case. In the positive electrode, a positive electrode active material layer containing a positive electrode material typified by a positive electrode active material is formed on the surface of a positive electrode current collector. In the negative electrode, a negative electrode active material layer containing a negative electrode active material is formed on the surface of a negative electrode current collector.
In a lithium-ion secondary battery as a typical example of the non-aqueous electrolyte secondary battery, a lithium composite oxide is used as a positive electrode material (positive electrode active material). This composite oxide is described in Patent Literature 1, for example.
Patent Literature 1 describes a non-aqueous electrolyte battery having a positive electrode active material with a Sn-containing oxide deposited on the surface of a lithium nickelate-based oxide (LiNiO2-based oxide).
In the non-aqueous electrolyte secondary battery, there has been a possibility that a crystal structure of a lithium composite oxide used for the positive electrode active material is destructed due to a kind of over-heating, overcharge or the like, to release contained oxygen which would trigger abnormal exothermic reaction, ignition and fuming.
The positive electrode active material described in Patent Literature 1 was aimed at preventing the surface state from deterioration in load characteristics by coating the surface of the lithium nickelate-based oxide with the Sn-containing oxide. However, a structure of the lithium nickelate-based oxide (lithium composite oxide) in the bulk remains as it is. That is, the above-mentioned problem of deterioration in safety associated with release of oxygen from the lithium nickelate-based oxide with destruction of the crystal structure remains unsolved in the process of charge-discharge cycling as well.
Patent Literature 1: JP 2003-257434-A